Coating of substrates, for example glass substrates, by means of cathode sputtering in plasma processes is a well-known process. Cathode sputtering can be performed conventionally or by use of a reactive gas. In the latter case, it is called reactive sputtering. A plasma is generated by means of a current or voltage source, which removes material from a target. The removed material is then deposited on the substrate. Before deposition, the atoms of the target material can combine with gas atoms or molecules in a reactive process.
In reactive processes, medium frequency (MF) generators may be used. Such generators typically operate at frequencies in the range from 10 to 500 kHz. Furthermore, pulsed generators may be used, in particular bipolar pulsed generators. Those generators have in common that they produce a periodic output signal. The periodic output signal is supplied to the plasma process. Typically, the output voltage of such generators is supplied to the plasma chamber by means of two electrodes, which are in turn used as cathode and anode. Both electrodes are connected to a target respectively. The generators may be freely oscillating generators or generators having a controlled frequency.
In particular, if reactive processes are being used, arcs can occur. If arcs occur, they may damage the substrate or the targets. Therefore, arcs should be avoided or, if an arc is detected, it should be quickly extinguished. To be able to extinguish an arc, it is necessary to detect arcs quickly and reliably. To detect arcs, the output signal of the power supply, i.e., a power supply signal, may be observed. In particular, if the output signal changes its value periodically, like in bipolar pulsed or MF plasma processes, the output signal at a specific time in a later period can be compared with a corresponding value at the same time in a previous period. Problems arise, when the output signal rises or falls very quickly. Especially, in bipolar pulse powered plasmas this is very relevant. If this happens, false arcs may be detected, i.e., an apparent arc may be detected even though there is no arc present. This can lead to an interruption of the plasma process, which is undesirable. On the other hand, if the threshold for arc detection is made too large to avoid these false arc detections, real arcs may be detected too late or not at all.